1. Field of the Invention
The field of art to which this invention pertains includes pressurized fluid control systems for multiple speed ratio powershift transmissions. More particularly, the control system is adapted to automatically shift the transmission one step from a higher speed ratio to a lower speed ratio at a first predetermined vehicle ground speed and thereafter shift back to said higher speed ratio at a second higher predetermined vehicle ground speed.
2. Description of the Prior Art
In front end loader applications, for example, during bucket loading at the pile portion of the work cycle, the vehicle is required to crowd the pile as well as pry or lift with the bucket. The pile is normally approached in second gear from a travel portion of the work cycle, with initial crowding and bucket work being done in second gear. During the second gear operation, in comparison with first gear operation less torque is available at the driving wheels which has a desirable effect of minimizing wheel spin. However, when additional hydraulic demands are placed on the engine, as a result of prying or bucket lifting operations, insufficient power remains for transmission to the driving wheels for effectively crowding the pile. Therefore, at this time the operator normally must manually shift to first gear and upon loading the bucket and backing out of the pile, he must then manually shift to second gear.
In order to relieve the operator from excessive shifting between first and second gears, it would be very desirable to have an automatic shift system so that, when the vehicle ground speed drops to a first predetermined speed, the transmission is automatically downshifted from second to first gear which in turn is followed by an automatic upshift when the vehicle ground speed thereafter exceeds a second higher predetermined speed. Therefore, a principal object of this invention is to provide a transmission shift control system that provides this automatic function.
Detailed descriptions of the transmissions regulated by this type of a control system may be found in U.S. Pat. No. 3,126,752 to Bolster, with control covers and the associated valves to which this invention pertains being found in U.S. Pat. No. 3,334,703 to Zeller and in U.S. Pat. No. 3,559,780 to Erdman.